1. Field of the Invention
This invention is directed to motor vehicle heat storage devices and, more particularly, to motor vehicle cabin heating systems which employ a rechargeable latent heat storage device, otherwise known as a heat storage battery, for providing rapid warm up of the engine and the cabin or passenger compartment of a motor vehicle.
2. Description of Related Art
In a motor vehicle with a typical liquid cooled internal combustion engine, the cabin or passenger compartment may be heated by means of a cabin heater which includes a liquid-to-air heat exchanger or heater core. Hot coolant from the engine cooling system is supplied to the heat exchanger, and air is blown over heat transfer elements of the heat exchanger to supply warm air to the cabin interior. The capacity of the heater to warm the vehicle cabin increases as the engine of the vehicle approaches its normal operating temperature. Thus, the engine must be operated for several minutes before it can provide heated coolant for warming the cabin.
In modern motor vehicles, particularly automobiles, even though the cabin climate control systems have achieved a high degree of sophistication in maintaining a desirable temperature in the compartment, a degree of discomfort continues to exist while waiting for engine warm up. As a corollary, there is a corresponding delay in effective operation of the vehicle's windshield defrosting system.
Latent heat storage devices, such as heat storage batteries, are available which are capable of storing, and releasing upon demand, a substantial quantity of thermal energy for a period of time up to a certain period of time. Such devices provide an auxiliary source of heat to the cabin interior and windshield defrosters, or to the engine for more rapid engine warm up.
Compact latent heat storage batteries which have the capability of retaining, for a period of time, a quantity of heat in a liquid/solid medium are described, for example, in U.S. Pat. Nos. 4,932,465 of Jun. 12, 1990 and 5,114,071 of May 19, 1992.
Such heat storage batteries employ the principle of latent heat conversion. Typically, they include a plurality of individual cells which contain a medium which has a substantially high capacity for storage of latent heat in a liquid form. The medium can be readily transformed from a liquid phase to solid phase to release such latent heat and can be recharged by reheating to liquid form.
The solid/liquid phase transition temperature of the medium is typically in excess of the desired cabin ambient temperature and less than the thermostatically regulated coolant temperature. As described in the above-identified patents, a typical medium consists of a barium hydroxide salt which is characterized by a low sensible heat capacity, a high latent heat capacity, and a melting temperature of about 78.degree. C.
Various systems have been proposed for use of such heat storage batteries for heating a cabin and/or more rapidly warming up the engine. Examples of engine warming systems using such heat storage batteries are shown in U.S. Pat. No. 5,101,801 to Schatz, issued Apr. 7, 1992. This patent discloses methods of heating an internal combustion engine by warming the air drawn into the engine with heat drawn from a heat storage means which stores heat from either the engine coolant or from the exhaust gas.
The Schatz patent further discloses that the heat storage means may be placed in a circuit with a cabin heater for the vehicle wherein a portion of the heat from the heat storage means is used to heat coolant passing through the cabin heater. In the systems disclosed in the Schatz patent, the coolant or medium warmed by the heat storage means first passes through a heat exchanger for supplying heat to the combustion air prior to flowing to the cabin heater, such that the amount of heat supplied to the cabin from the heat storage means is reduced by the amount of heat diverted to the combustion air.
It is also known to provide a cabin heating system wherein engine coolant is conveyed from a heat storage battery directly to the heat exchanger for a vehicle cabin heater and then back to the engine. While such a system is quite effective for quickly heating the vehicle cabin as well as the engine, it has been found that once the stored heat has been discharged, the heat storage medium begins to absorb heat from the coolant as the coolant temperature increases which, in turn, inhibits the rate at which the coolant temperature is increased to its final operating temperature.
In other words, due to the nature of the heat transfer medium, the heat storage battery begins to recharge itself from heat supplied by the engine coolant at an inopportune time; namely, just at the time that the coolant becomes able to take over the heating of the passenger cabin, but prior to the time when the engine and the cabin are fully warmed and window defrosting or demisting has been fully accomplished.
There accordingly exists a need for improved controls and circuitry to optimize the use of a heat storage battery in combination with a motor vehicle heating system.